Question

For each of these reactions, identify the acid and base among the reactants, and state if the acids and bases are Lewis, Arrhenius, and/or Bronsted-Lowry: (a) ${\mathrm{PCl}}_4^{+}+{\mathrm{Cl}}^{-}⟶{\mathrm{PCl}}_5$ (b) ${\mathrm{NH}}_3+{\mathrm{BF}}_3⟶{\mathrm{H}}_3{\mathrm{NBF}}_3$ (c) ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}+{\mathrm{H}}_2\mathrm{O}⟶{\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{OH}\right]}^{2+}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

Short answer

(a) Acid: ${\mathrm{PCl}}_4^{+}$ (Lewis acid); Base: ${\mathrm{Cl}}^{-}$ (Lewis base) (b) Acid: ${\mathrm{BF}}_3$ (Lewis acid); Base: ${\mathrm{NH}}_3$ (Lewis base) (c) Acid: ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ (Bronsted-Lowry acid); Base: ${\mathrm{H}}_2\mathrm{O}$ (Bronsted-Lowry base)

Step-by-step solution

Identify the acid and base among the reactants

For this reaction, ${\mathrm{PCl}}_4^{+}$ is donating a ${\mathrm{Cl}}^{-}$ to the reactant, which means it is acting as an acid, and ${\mathrm{Cl}}^{-}$ is accepting it, so it is acting as the base.

Determine the type of acids and bases

Since ${\mathrm{PCl}}_4^{+}$ is donating a ${\mathrm{Cl}}^{-}$, it is a Lewis acid. Since ${\mathrm{Cl}}^{-}$ is accepting a ${\mathrm{Cl}}^{-}$, it is a Lewis base. The reaction does not involve donating or accepting protons (H${}^{+}$), so this reaction does not involve Arrhenius or Bronsted-Lowry acids and bases. (b) ${\mathrm{NH}}_3+{\mathrm{BF}}_3⟶{\mathrm{H}}_3{\mathrm{NBF}}_3$

Identify the acid and base among the reactants

In this reaction, ${\mathrm{NH}}_3$ donates a lone pair of electrons to ${\mathrm{BF}}_3$. Therefore, ${\mathrm{NH}}_3$ is acting as the base and ${\mathrm{BF}}_3$ is acting as the acid.

Determine the type of acids and bases

Since this reaction involves the transfer of a lone pair of electrons, both the acid and base in this reaction are Lewis acids and bases. There is no proton (H${}^{+}$) transfer, so there are no Arrhenius or Bronsted-Lowry acids and bases in this reaction. (c) ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}+{\mathrm{H}}_2\mathrm{O}⟶{\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{OH}\right]}^{2+}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

Identify the acid and base among the reactants

In this reaction, ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ is donating a proton (H${}^{+}$) to the water molecule (H${}_2$O). Hence, ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ acts as the acid and water acts as the base (H${}_2$O).

Determine the type of acids and bases

Since the reaction involves the transfer of a proton (H${}^{+}$), both the acid and base in this reaction are Bronsted-Lowry acids and bases. As there are no lone pairs of electrons being transferred, there are no Lewis acids or bases in this reaction. The reaction involves water, but the dissociation of ions is not evident, so there are no Arrhenius acids and bases in this reaction.

Key concepts

Lewis acids and bases

Lewis acids and bases revolve around the idea of electron pair interaction. A Lewis acid is a chemical species that can accept an electron pair, while a Lewis base donates an electron pair. This concept is broader than the traditional definition of acids and bases because it does not rely on the presence of protons.

For example, in the reaction ${\mathrm{NH}}_3+{\mathrm{BF}}_3⟶{\mathrm{H}}_3{\mathrm{NBF}}_3$, ${\mathrm{BF}}_3$ acts as the Lewis acid since it accepts an electron pair from ${\mathrm{NH}}_3$. In contrast, ${\mathrm{NH}}_3$ is the Lewis base because it donates an electron pair to form the bond.

Understanding Lewis acids and bases is crucial because many chemical reactions, especially those involving metal complexes, occur through electron pair donation and acceptance.

Bronsted-Lowry acids and bases

In the Bronsted-Lowry framework, acids are defined as proton donors, and bases as proton acceptors. This extension of the Arrhenius concept allows for a more comprehensive understanding of acid-base reactions, especially in non-aqueous solutions.

Consider the reaction ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}+{\mathrm{H}}_2\mathrm{O}⟶{\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{OH}\right]}^{2+}+{\mathrm{H}}_3{\mathrm{O}}^{+}$. Here, ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ serves as the Bronsted-Lowry acid because it donates a proton to water, which acts as the Bronsted-Lowry base by accepting the proton.

This concept is essential for understanding reactions where protons are exchanged between molecules, crucial in biological systems and industrial processes.

chemical reactions

Chemical reactions are processes where reactants are transformed into products. These involve breaking existing chemical bonds and forming new ones, frequently yielding energy changes.

Acid-base reactions are one type of chemical reaction. They are characterized by the transfer of protons or electron pairs. Each type of acid-base reaction depends on the models used, such as Lewis or Bronsted-Lowry.

Understanding chemical reactions requires grasping these models' nuances, as they dictate how molecules interact and change during reactions, influencing everything from reaction speed to product formation.

proton transfer

Proton transfer is a fundamental process in acid-base chemistry, particularly within the Bronsted-Lowry framework. In these reactions, protons (${\text{H}}^{+}$ ions) are transferred from an acid to a base.

For instance, in the reaction ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}+{\mathrm{H}}_2\mathrm{O}⟶{\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\mathrm{OH}\right]}^{2+}+{\mathrm{H}}_3{\mathrm{O}}^{+}$, a proton is transferred from the aluminum complex to water, highlighting water's role as a base.

This mechanism is significant in many reactions across various fields, including biochemistry, where enzymes often catalyze reactions through proton transfers to form essential biological molecules.

electron pair donation

Electron pair donation is a key concept in Lewis acid-base theory. Here, an electron pair donor, the Lewis base, provides electrons to the Lewis acid, which accepts them.

In the reaction ${\mathrm{NH}}_3+{\mathrm{BF}}_3⟶{\mathrm{H}}_3{\mathrm{NBF}}_3$, ${\mathrm{NH}}_3$ donates a lone pair of electrons to ${\mathrm{BF}}_3$, illustrating this concept beautifully. This forms a new coordinate covalent bond, cementing ${\mathrm{NH}}_3$ as a Lewis base and ${\mathrm{BF}}_3$ as a Lewis acid.

Understanding electron pair donation is crucial for comprehending many reactions, especially those involving coordination compounds, which play vital roles in catalysis and materials science.